The present invention generally relates to a biaxially oriented polyester film with improved dyeability, and more particularly, to a polyester film excellent in dyeability which is prepared by melt-blending polyethylene terephthalate with polytrimethylene terephthalate.
Polyethylene terephthalate molded materials have been widely used in various industrial fields as a plastic material for fiber, film and the like. Among the molded articles, polyethylene terephthalate films composed of aromatic dicarboxylic acids and glycols have excellent mechanical properties including heat resistance, tensile strength, Young""s modulus, elastic recovery and impact resistance, dimensional stability and electrical insulating strength, and are being broadly used in a number of agricultural and industrial applications such as magnetic recording tape, photographic film, insulating material and base film of coating application. Besides, polyethylene terephthalate films are being frequently utilized in food and consumer packaging applications due to their excellent chemical properties including chemical resistance, weather resistance and water resistance, transparency, flavor retentivity, gas barrier property.
Typically, an industrial process for producing polyethylene terephthalate films is carried out in a known manner, for example, by melt-extruding a polyethylene terephthalate polymer, cooling the extruded polymer in a cooling drum to obtain an amorphous sheet, and stretching and heat setting the amorphous sheet into a biaxially oriented film.
An industrial process for producing the polyethylene terephthalate polymer used to produce polyethylene terephthalate films includes a direct esterification reaction in which dicarboxylic acids including mainly terephthalic acid are esterified with ethylene glycols at a temperature of 200 to 280xc2x0 C. under atmosphere or vacuum, or an ester interchange reaction (i.e., transesterification) in which dimethylcarboxylates including mainly dimethyl terephthalate are transesterified with glycols including mainly ethylene glycol in the presence of a transesterification catalyst at 140 to 240xc2x0 C., thereby obtaining bis(xcex2-hydroxyethyl)terephthalates and low-molecular condensates thereof (i.e., esterified products); and a polycondensation stage in which the esterified products obtained in the direct esterification or transesterification stage are subjected to polycondensation in the presence of a polycondensation catalyst with heating at 260 to 300xc2x0 C. under high vacuum.
Polyethylene terephthalate is known to be of great value in the industrial aspect due to its excellent properties as described above. Nonetheless, polyethylene terephthalate is limited in application because it has few active functional groups in the molecular. For example, polyethylene terephthalate is thus inferior in adhesiveness, dyeability, and moisture-absorbing and antistatic properties to other polymers, namely, polyamides, acetates and celluloses.
In an attempt to improve such poor properties of polyethylene terephthalate, especially, dyeability, there are suggested several conventional methods including, for example, (1) adding a copolymer component to the aromatic polyethylene terephthalate decrease the crystallinity and increase the dispersability of the dye, or (2) adding a compound having an affinity to the dye to strengthen the bonding with the dye.
However, those conventional methods are not so much effective and rather disadvantageous in that the polyethylene terephthalate copolymers decrease the softening temperature of polyethylene terephthalate and causes discoloration or nonuniformity of the mixture, thereby deteriorating all or some of the above-mentioned excellent mechanical, physical or chemical properties of the polyethylene terephthalate.
In view of the above problem, several methods have been suggested to improve the dyeability of polyethylene terephthalate without addition of some copolymer components.
A first method is to use a dye having a relatively small size of molecules, which dye is poor in bonding into the polyethylene terephthalate film in washing. Another method involves adding carriers to the aqueous dye bath with a rise of production cost. In the third method, the formed articles are pretreated with a semi-solvent to have a double-layered (sheath-core) structure, the outer layer of which is more susceptible to dyeing and provides ununiformed dyeing to thickness of film. Besides, a solution dyeing method has been proposed in which the final products(In case of fiber is texture) are dyed with a melt blend of the dye and the polyethylene terephthalate polymer. The solution dyeing method may be effective in dyeing but actually is inapplicable to a multiproduct process on a small scale.
It is an object of the present invention to solve the problem with the dyeability of a polyethylene terephthalate and to provide a polyethylene terephthalate film with improved dyeability that is prepared by blending polytrimethylene terephthalate with a polymer of polyethylene terephthalate alone without addition of some copolymer component.
To achieve the object of the invention, there is provided a biaxially oriented film prepared by melt-blending 55 to 90 wt. % of polyethylene terephthalate with 10 to 45 wt. % of polytrimethylene terephthalate.